Author Topic: Understanding bipolar transistors  (Read 1128 times)

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dsharp02

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Understanding bipolar transistors
« on: March 08, 2016, 12:16:15 pm »
Apologies if my questions are stupid and for my crappy notation.  I'm very much a newbie and mainly a software developer.

I'm slowly grinding my way through TAoE, and it occurred to me that one way of thinking about bipolar transistors would that increasing base current lowers collector-emitter resistance.  This makes sense to me but I think it's probably also wrong, because then for a fixed Ibe (and therefore a fixed resistance) increasing the Vce would increase Ice in proportion to the voltage.  My understanding is that for a fixed Ibe the Ice is also fixed (except for something call the Early effect, dunno haven't read this section enough times to understand yet).  So increasing voltage would not change Ice.  This seems to imply that the resistance from collector to emitter would need to vary in proportion to Vce to limit the current to beta * Ibe.

Example: For a fixed 1mA of base current (assume beta = 100).  Then the resistance from collector to emitter would be Vce / 100ma right?

Or does the fact that there is no single fixed value mean that it is meaningless to talke about the resistance between collector and emitter (Rbe?).  Would this resistance be the same as (I've heard the term somewhere) the dynamic resistance?

Or have I gotten lost and wandered off into the wilderness?

I'm just trying to get my mental model for bipolar transistors straight.

uncle_bob

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Re: Understanding bipolar transistors
« Reply #1 on: March 08, 2016, 12:44:40 pm »
Apologies if my questions are stupid and for my crappy notation.  I'm very much a newbie and mainly a software developer.

I'm slowly grinding my way through TAoE, and it occurred to me that one way of thinking about bipolar transistors would that increasing base current lowers collector-emitter resistance.  This makes sense to me but I think it's probably also wrong, because then for a fixed Ibe (and therefore a fixed resistance) increasing the Vce would increase Ice in proportion to the voltage.  My understanding is that for a fixed Ibe the Ice is also fixed (except for something call the Early effect, dunno haven't read this section enough times to understand yet).  So increasing voltage would not change Ice.  This seems to imply that the resistance from collector to emitter would need to vary in proportion to Vce to limit the current to beta * Ibe.

Example: For a fixed 1mA of base current (assume beta = 100).  Then the resistance from collector to emitter would be Vce / 100ma right?

Or does the fact that there is no single fixed value mean that it is meaningless to talke about the resistance between collector and emitter (Rbe?).  Would this resistance be the same as (I've heard the term somewhere) the dynamic resistance?

Or have I gotten lost and wandered off into the wilderness?

I'm just trying to get my mental model for bipolar transistors straight.

Hi

If the resistance between collector and emitter varied with base current, then doubling the voltage on the collector would double the current. This does not happen, so that model is not quite right.

The most basic model of a bipolar transistor is a current controlled current sink. The current in the collector is (relatively) independent of collector voltage with a constant base current.

All models break down as you approach limits. At some point the voltage on the collector gets high enough to break down the device. It may also get high enough to change the temperature of the device or even to melt the poor thing. At the other end, when the voltage on the collector to base gets below 0.7V (Vce < 1.4V) things begin to change a bit.

Yes it's complicated. Sorry about that.

Bob

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Re: Understanding bipolar transistors
« Reply #2 on: March 08, 2016, 02:34:26 pm »
Youtube and others have videos, and just googling transistor basics or transistor
applications will get you a ton of ref material.

Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer

amyk

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Re: Understanding bipolar transistors
« Reply #3 on: March 08, 2016, 02:38:31 pm »
A simple way to think about it is that it limits Ice, so when Ice is small it does act somewhat like a resistor with a close-to-linear I-V curve (the saturated region), but after that the I-V curve takes on a much shallower slope, which is still not completely flat; this is due to the Early effect.

T3sl4co1l

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Re: Understanding bipolar transistors
« Reply #4 on: March 09, 2016, 02:05:25 am »
It's not a resistor, it's a constant current.  Even better than hFE is noting that:
Ib = Ise * (exp(Vbe/Vth) - 1)
Ic = Isc * (exp(Vbe/Vth) - 1)

It's like a current mirror, where the collector current mirrors the current of the base, but the collector voltage isn't confined to ~0.6V (Vbe) in the process, but can vary freely.

Ise is the 'saturation current' of the emitter (so, B-E junction), and Isc the collector.  You can see clearly that, all else being equal, hFE = Isc / Ise.  But these parameters are independent and can vary, which is why hFE itself varies so dangerously.

Both of these only count for the linear range, where Vce > Vce(sat).  In saturation, you can think of it as forward-biasing the B-C junction, so the low collector voltage acts to shunt away base current, limiting its "on-ness".  The B-C junction has a different built-in potential, so Vce(sat) is positive, usually some 100mV or so.  (When inverted, i.e., using emitter as collector, the potential goes the other way; Vec(sat) still isn't negative -- that would violate conservation laws -- but it can be ~mV instead!)

So as long as you remember that a BJT is composed of two diodes, one which acts as a sensor (reading Vbe), and one which acts as a constant current sink (where Ic depends exponentially upon Vbe), you can very well describe operation from linear range to saturation.

Exponentials are rather inconvenient for basic analysis, so you can make some empirical assumptions, like Vbe = 0.6V and hFE ~= 200 (or whatever the datasheet ballparks), to facilitate things.

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
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